Non Current Assets

On 31 January 2008,Westvale Co disposed of a building for 450000.The building was accounted
for using revaluation model. At the date of disposal,after deduction of accumulated depreciation of 90000,the building had a carrying amount of 310000.It also had a revaluation surplus in equity of 30000.
What was the Westvale Co’s profit on disposal of the building for inclusion in the statement of profit or loss for the year ended 31 December 2008?
Sir the i know that the revaluation is not included in profit but sir we know the disposal proceeds minus carrying amount is 450000-310000=140000 but in book the depreciation is added back to the carrying amount 410000+90000=500000 and the profit is 50000 .
Sir why the depreciation is added back